Summary of Investments - Ltd partnership Sec lending Special deposits (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Nature and activities of the VIEs and the effect on financial statements
Carrying value to loss in relation to the activities of VIEs	$ 63,586,000	$ 45,540,000
Maximum exposure to loss in relation to the activities of the VIEs	63,586,000	45,540,000
Special deposits and securities lending
Amortized cost	98,705,000
Securities loaned to third parties at fair value	93,255,000	0
Collateral received	80,013,000	0
Collateral fair value	16,502,000
Payable under securities lending agreements	80,013,000	$ 0
Corporate debt securities
Special deposits and securities lending
Payable under securities lending agreements	$ 80,013,000